1. Field of the Invention
The present invention relates to a structure of a carbon molecular beam source provided in a molecular beam epitaxial growth apparatus and particularly to a structure of graphite filament which operates as a carbon molecular beam source.
As a p-type dopant for III-V compound semiconductor, beryllium (Be) has been generally used. However, in these years, a p-type base of a heterojunction bipolar transistor (HBT) which has been produced by the molecular beam epitaxy (MBE) method as a high speed device provides a problem that a concentration profile is disturbed remarkably in the base layer due to diffusion of Be because a base layer is thin and concentration of Be is high. Therefore, carbon (C) which is more difficult to result in diffusion than Be is attracting more attention as the p-type dopant.
Carbon vapor is usually generated by heating graphite resistively with a current. In this case, it is extensively required that an amplitude of the heating current is in the practical range and carbon beam intensity is stabilized from the point of view of manufacturing a device.
2. Description of the Related Art
In order to generate carbon beam in a molecular beam epitaxial growth method, carbon vapor is usually generated by resistively heating of a graphite filament provided in a doping cell of a molecular beam epitaxial growth apparatus (hereinafter, called a MBE growth apparatus). FIG. 1 is a perspective view of a conventional graphite filament which operates as a carbon molecular beam source.
As shown in FIG. 1, a filament 10 comprises a heating portion 6 and current terminals 3 provided at both ends thereof. A material used is a sintered graphite or pyrolytic graphite. The heating portion 6 is sized, for example, as 1 mm in thickness, 2 mm in width and 50 mm in length.
When sintered graphite is selected as the material, the filament 10 is cut in the direction selected freely from a block of the sintered graphite. Meanwhile, when pyrolytic graphite is selected, a generally available material is thinner in the direction of c-axis and is formed as a plate spreading in the plane perpendicular to the c-axis. Therefore, the filament 10 has a structure allowing power feeding in the plane perpendicular to the c-axis. Intensity of carbon molecular beam can be controlled by controlling a current fed to the filament 10.
However, since resistivity of sintered graphite is not anisotropic and is about 10.sup.-2 ohm.multidot.mm, while resistivity of pyrolytic graphite in the direction perpendicular to the c-axis is about 4.times.10.sup.-3 ohm.multidot.mm, a heating current of the filament 10 has a considerably large value. This is a first problem. That is, graphite must be set to a temperature of about 2000.degree. C. in order to generate carbon molecular beam by heating graphite with a current. For this purpose, the filament consumes the power of about 300 W/cm.sup.2. In the case of sintered graphite, when the heating portion 6 of the filament 10 is sized as 1 mm in thickness, 2 mm in width and 50 mm in length (current direction), the filament 10 shows resistance of 0.25 .OMEGA. and current of 35 A. On the other hand, when the same filament 10 is formed of pyrolytic graphite, a current is fed in the direction perpendicular to the c-axis, resistance becomes 0.1 .OMEGA. and current is about 55 A. The filament 10 explained above is used for a carbon molecular beam source cell of a small size MBE growth apparatus which the distance between the growth substrate and the carbon molecular beam source cell is about 20 cm. If this filament 10 is used for carbon molecular beam source of a large size MBE growth apparatus resulting in the distance of about 60 cm, the carbon molecular beam intensity which is about one order of magnitude larger than that used for the small size apparatus is required. In this case, consumption of the filament 10 due to evaporation of carbon also becomes about ten times larger and therefore thickness of the filament must be ten times thicker than that for a small size MBE growth apparatus if the filament area is constant. When the heating portion 6 is formed of the pyrolytic graphite sized as 10 mm in thickness, 2 mm in width and 50 mm in length, a current becomes 173 A and this value is practically too large.
Of course, it is possible to provide a filament having the area increased by 10 times, but power consumption is also increased up to 10 times in this case. From this point of view, it is not a practical method.
The second problem of a conventional graphite filament is that if filament resistance changes, intensity of molecular beam also changes because intensity of carbon molecular beam is controlled by a filament current. That is, a resistance value of the filament 10 changes with evaporation of carbon. Accordingly, if a current is controlled to a constant value, power consumption of the filament 10 changes and temperature of the filament 10 also changes. As a result, intensity of carbon molecular beam changes. For instance, under the ordinary application condition, the filament 10 mentioned above (the heating portion 6 has the sizes of 1 mm in thickness, 2 mm in width and 50 mm in length) shows reduction of thickness at a rate of about 1 .mu.m/h. Therefore, resistance increases at a rate of about 0.1%/h, while power consumption also increases. Temperature of the filament rises to 2000.57.degree. C. from 2000.degree. C., while carbon vapor pressure is almost proportional to 10.sup.-40000 /T (T is the absolute temperature of the filament). Thereby, intensity of carbon molecular beam increases at a rate of about 1%/h.